1. Field of the Invention
The present invention generally relates to micromachined sensing devices and methods. More particularly, this invention relates to devices and methods that integrate multiple sensing techniques and elements on a single substrate.
2. Description of the Related Art
A process and design for fabricating resonant mass flow and density sensors using a silicon micromachining technique are disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al. As used herein, micromachining is a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer), or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. In the process disclosed by Tadigadapa et al., wafer bonding and silicon etching techniques are used to produce a suspended silicon tube on a wafer. The tube is vibrated at resonance, by which the flow rate and density of a fluid flowing through the tube can be determined.
While well suited for the function of sensing flow rates and densities of fluids, it would be advantageous if other properties of a fluid could be determined on the same wafer, yielding an integrated device that is more flexible in meeting system needs.
The present invention provides a sensing device that comprises a micromachined tube on a substrate, such as of the type disclosed in U.S. Pat. No. 6,477,901 to Tadigadapa et al. for resonant sensing of mass flow and density of a fluid flowing through the tube. The sensing device of this invention further incorporates on the same substrate at least a second micromachined tube configured for sensing another property of the fluid, such as pressure, viscosity and/or temperature.
The sensing device of this invention comprises a first tube micromachined on a substrate and comprising a fluid inlet, a fluid outlet, and a freestanding portion between the fluid inlet and the fluid outlet so as to define a continuous passage for a fluid flowing through the first tube. The freestanding portion is spaced apart from a first surface of the substrate, and means is provided for vibrating the freestanding portion at a resonant frequency thereof. In addition, means is provided for sensing movement of the freestanding portion of the first tube. A second micromachined tube is fabricated on the substrate. According to a first embodiment of the invention, the second tube is in series with the first tube such that the fluid flows through both the first and second tubes. The second tube may have a bridge portion spaced apart from a second surface of the substrate, and that is operable to deflect toward and away from the second surface in response to a change in pressure of the fluid flowing through the second tube, such that sensing the proximity of the bridge portion to the second surface of the substrate is indicative of the pressure of the fluid flowing through the first and second tubes.
According to a second embodiment of the invention, the sensing device may further comprise a third micromachined tube on the substrate and in series with the first and second tubes, such that the fluid flows through the second, first and third micromachined tubes, respectively. Similar to the second tube, the third tube may have a bridge portion spaced apart from the substrate and operable to deflect toward and away from the substrate in response to a change in pressure of the fluid flowing through the third tube. By also sensing the proximity of the bridge portion of the third tube to the substrate, the viscosity of the fluid can be determined based on the relative pressures sensed with the second and third tubes.
According to third and fourth embodiments of the invention, the second tube comprises a cantilevered portion spaced apart from a second surface of the substrate member. In the third embodiment, the cantilevered portion is operable to sense motion of the sensing device, while in the fourth embodiment of the invention the cantilevered portion thermally communicates with the first tube for sensing the temperature of the fluid flowing through the first tube. In this embodiment, the cantilevered portion is vibrated at a resonant frequency thereof, and changes in temperature of the fluid flowing through the first tube is sensed by detecting changes in the resonant frequency of the cantilevered portion.
Various additional features can be incorporated into the sensing device of the present invention. For example, the device may be equipped with a cap hermetically bonded to the substrate so as to define a hermetically-sealed evacuated enclosure containing the first and second tubes. If the second tube is operable as a temperature sensor, the cap can be adapted to shield the second tube from thermal radiation, or shield all but certain wavelengths of interest, e.g., infrared (IR). As another example, one or more metal layers can also be incorporated into the sensing device, such as for sensing the temperature of the fluid by sensing changes in electrical resistance of a metal layer, or for sensing the electrical conductivity, dielectric constant and pH of the fluid with two metal layers spaced apart and in contact with the fluid. Yet another feature that can be integrated with the sensing device is one or more stress-sensing members configured to be substantially similar to the first and/or second tubes but through which the fluid does not flow, so that movement of the stress-sensing member caused by extraneous sources can be sensed and used to compensate the output obtained from the first and/or second tubes. Finally, the sensing device can be provided with electrodes contacting portions of the first tube at or near the fluid inlet and the fluid outlet thereof, and means for flowing a current between the electrodes and through the first tube so as to heat and maintain the fluid flowing therethrough at a desired temperature. In this manner, more accurate density, viscosity and/or flow measurements of the fluid can be obtained.
In view of the above, it can be seen that the present invention provides for the integration and design of various sensing techniques in a resonant micromachined tube and process. Such devices include pressure, temperature, viscosity, and optical/IR sensors, which make possible a variety of improved chemical or biochemical sensors and motion sensors. Further improvements are achieved by integrated stress compensation into the micromachining process and design. By incorporating on the same wafer multiple structures capable of sensing multiple properties of a fluid, the present invention is able to provide an integrated device that is more capable of meeting various requirements of a fluid analysis system.
Other objects and advantages of this invention will be better appreciated from the following detailed description.